1. Field of the Invention
The present invention generally relates to a driving circuit and, more particularly to a light emitting diode (LED) driving circuit.
2. Description of the Related Art
Referring to FIG. 7, a conventional LED driving circuit 70 is adapted to drive a LED load 300. The LED driving circuit 70 includes a transformer T1, a rectifying filtering circuit 72, a pulse width modulation integrated circuit (PWM IC) 74, a feedback circuit 76, and a constant current circuit 78. The transformer T1 has a primary side and a secondary side. The rectifying filtering circuit 72 is electrically coupled to the secondary side of the transformer T1 and used to rectify and filter alternating current (AC) power outputted from the secondary side of the transformer T1 and output a direct current (DC) voltage from an output terminal thereof. The PWM IC 74 is electrically coupled to the primary side of the transformer T1. The feedback circuit 76 has an inverting input terminal and a non-inverting input terminal. The inverting input terminal is electrically coupled to the rectifying filtering circuit 72 to receive the DC voltage. The non-inverting input terminal receives a constant reference voltage Vref1. The feedback circuit 76 controls the PWM IC 74 according to both the DC voltage and the reference voltage Vref1 respectively received by the inverting input terminal and the non-inverting input terminal thereof. The LED load 300 is electrically coupled between the output terminal of the rectifying filtering circuit 72 and an output terminal of the constant current circuit 78. The constant current circuit 78 includes a three-terminal regulator U2, a field effect transistor Q1, and resistors Ra, Rb. An output terminal of the three-terminal regulator U2 receives a DC voltage VCC through the pull-up resistor Rb. A gate terminal of the field effect transistor Q1 is electrically coupled to the output terminal of the three-terminal regulator U2, a drain terminal thereof is electrically coupled to an input terminal of the three-terminal regulator U2, and a source terminal thereof is used as the output terminal of the constant current circuit 78. The resistor Ra is electrically coupled between a ground terminal and the input terminal of the three-terminal regulator U2.
Furthermore, when there is a need to perform an external PWM dimming applied to the LED load 300, a dimming circuit 79 is electrically coupled to the ground terminal of the three-terminal regulator U2 of the constant current circuit 78. The dimming circuit 79 includes an operational amplifier U1 and a field effect transistor Q3. An inverting input terminal and a non-inverting input terminal of the operational amplifier U1 respectively receive a pulse width modulation signal PWM and a reference voltage Vref2. An output terminal of the operational amplifier U1 is electrically coupled to a gate terminal of the field effect transistor Q3. The field effect transistor Q3 is electrically coupled between the ground terminal of the three-terminal regulator U2 and a ground potential.
However, the LED driving circuit 70 utilizes the three-terminal regulator U2, the field effect transistor Q1, and the resistor Ra to produce a constant current output, and the outputted DC voltage is constant rather than varied with the LED load 300, so that a relatively large power loss occurs on the field effect transistor Q1. In addition, an additional field effect transistor Q3 is needed to perform the external PWM dimming.